Materials engineering continues to develop new plastics and ceramics, many of which may be appropriate for use in radio frequency (RF) devices. Low loss dielectric materials are measured to ascertain their qualities and potential applications. A number of techniques and apparatus exist to assist in the measurements. One such system utilizes the cavity resonance technique to measure the dielectric constants and loss tangents of materials. The materials measured may include plastic and ceramic materials used in RF devices. For example, antennas may require that low loss dielectric materials be used in the construction of antenna feed and radome systems.
The cavity resonance technique employs a resonant cavity in which the sample to be measured is placed. The resonant cavity is designed in the standard TE (transverse electric) or TM (transverse magnetic) mode of propagation of the electro-magnetic fields, for example. The technique is based on an accurate model of the resonant frequency and volumetric absorption characteristics of the cavity. The sample completely fills a slice through the waveguide cross section (e.g., rectangular or cylindrical) so that it can be accurately modeled as a short length of filled waveguide transmission line. With such an accurate model, the values of the dielectric constant and loss tangent can then be varied within the model to provide correspondence with the cavity measurements. The final values required to produce a match are the measured values. Changes in the center frequency and width of the resonance transmission based on the insertion of the sample provide information that may be used to calculate the dielectric constant. Changes in the Q-factor (ratio of energy stored to energy dissipated) are used to estimate the dielectric loss. See Vankatesh, M. S. and Raghavan, G. S. V. 2005. “An overview of dielectric properties measuring techniques.” Canadian Biosystems Engineering/Le génie des biosystèmes au Canada 47:7.15-7.30, 7.18-7.19.
For general information concerning waveguides and cavity resonators, see generally D. Jeffries, “Waveguides and Cavity Resonators,” Jan. 14, 2005. The precise calculations employed in analyzing the measurements and estimating the dielectric constant and loss tangents of materials, for example, are known to one of ordinary skill in the art. For an example of estimating the quality factor (Q) and resonant frequency (f0) of a microwave cavity based on observations of a resonance curve on an equally spaced frequency grid, see Coakley et al., “Estimation of Q-Factors and Resonant Frequencies,” IEEE Trans. of Microwave Theory and Techniques, Vol. 51, No. 3, pp. 862-868, March 2003.
An example conventional TE0,n waveguide cavity measurement system 10 is shown in FIG. 1. The system 10 generally includes a metal cylinder (that is, sidewalls) 12 with a moveable endplate 14 on the bottom (for fine tuning of a cylindrical cavity 22 and insertion of a dielectric sample 16), a fixed endplate 18 on the top and coupling apertures (that is, coupling holes or coupling receptacles) 20, also on the top and passing through the fixed endplate 18. The coupling holes 20 permit a signal to pass through the cavity 22 where the dielectric sample 16 sits. The dielectric sample 16 is in the shape of a cylindrical disc. For this example, assume the cavity 22 is designed to resonate in the low-loss TE0,n modes. In the conventional system 10 of FIG. 1, additional components are also shown, including: a surrounding layer of water 24 and temperature sensing probe 26 for thermally isolating the cavity 22 and measuring the temperature of the water 24 (the water jacket 24 may be connected to a water bath that controls its temperature); a detachable tuner assembly 28, tuner position sensing micrometer 30 and motor-driven micrometer 32 for finely-tuned moving of the moveable endplate 14; and a helical waveguide 34 that lines the interior wall of the cavity 22 (as further explained below). See J. Baker-Jarvis, R. G. Geyer, J. H. Grosvenor, Jr., et al., “Dielectric Characterization of Low-loss Materials—A Comparison of Techniques”, IEEE Trans. on Dielectrics and Electrical Insulation, Vol. 5 No. 4, pp. 571-577, August 1998.
Specific cavity modes can be observed. However, generally the cavity is overmoded and other, undesirable cavity modes are free to resonate unless otherwise suppressed. The unwanted cavity modes can interfere with the otherwise accurate measuring process.
One conventional method for suppressing the unwanted modes is to use fine conducting wire to form a helically-wound wall over the interior length of the cylindrical cavity. See, e.g. Baker-Jarvis et al.; Janezic et al., “Relative Permittivity and Loss Tangent Measurement using NIST 60 mm Cylindrical Cavity,” National Institute of Standards and Technology (NIST) Special Publication 260-159, August 2005, p. 4. The helical waveguide 34 (see FIG. 1) allows only azimuthal wall currents and permits only the desired TE0,n waveguide modes to propagate, thereby attenuating unwanted modes.